The pope's announcement that he will resign has taken many people by surprise, but experts on aging say perhaps it shouldn't.

They say that as people live longer, the physical and mental challenges of old age are catching up with more of those in positions of power. Many are choosing to step down instead of continuing in jobs traditionally held until death.

Justices appointed to the U.S. Supreme Court used to serve until death. But since 1955, 21 justices have retired and only one, Chief Justice William Rehnquist, died in office.

The Netherlands' Queen Beatrix just announced she would pass the crown to her son in April. And 20 percent of U.S. senators are now 70 or older.